1. Field of the Invention
This invention relates to a circuit multiple transmission system for transmitting speech and facsimile signals at high efficiency.
2. Description of the Related Art
As demand for communications has increased in recent years, request for high-efficiency transmission of voice over telephone, which is the majority of the communication demands, is on the rise. Answers to this request have been realized as a DSI (Digital Speech Interpolation) system, which improves the voice transmission efficiency by sending only the voice with sound by utilizing that the rate of voice with sound of voice over telephone is at most 40%, and a various kinds of speech encoders, which encode a speech signal in terms of number of bits less than the conventional PCM method.
An effective means for improving the line efficiency in international lines such as satellite communications and submarine cables has been realized as a DCME (Digital Circuit Multiple Equipment), which is a combination of the DSI system and the speech encoder. In the DCME using the ADPCM (Adaptive Differential Pulse Code Modulation) method as the speech encoding method, it is possible to increase the line efficiency about two and half times by the DSI process and about two times by the speech encoding, i.e. about five times in total, compared to the conventional case where a speech signal is sent as digital data at 64000 b/s by the PCM encoding method. Namely, assuming that all telephone line input signals are speech signals, the DCME by the ADPCM-DSI method can accommodate transmission lines about five times the PCM telephone lines of 64000 b/s, in terms of which the transmission circuit capacity as telephone line input is calculated. Of course, if telephone lines more than those are input, or if the voice activity of the input telephone lines is higher than expected, the transmission circuit capacity of the DCME would be in-sufficient, namely, there would be no remaining transmission lines to be allocated for the head portion of a speech, which is part of sound signal, so that the head portion of the speech will be cut off to deteriorate the conversation quality.
On the other hand, terminals to be connected to the telephone lines are exemplified by telephones and facsimiles. With the recent rapid spread of facsimiles, it cannot be presumed that in the DCME of the ADPCM-DSI method, every input signal is a voice signal. As a result, it would be Impossible to accommodate the input telephone lines five times the transmission circuit capacity so that circuit efficiency cannot be increased as expected. This is true because the signal of the trunk channel in facsimile communication is active in the majority of communication time so that the voice activity, which is essential for DSI, is sharply increased along with the rate of facsimile signals.
In an effort to solve this problems, facsimile signal base band transmission technology has currently been introduced which improves the transmission efficiency of facsimile signals by demodulating facsimile signals, i.e. voiceband data signals, for transmission as base band data and by modulating the voiceband data signal again at the receiver's system. A system for high-efficiency transmission of voice/facsimile signals utilizing this transmission technology and the above-mentioned DCME in combination is now put on the market.
FIG. 8 shows this conventional type system, which is disclosed in "Development of Facsimile Signal Demodulation Multiple Equipment" by Mitsuhiro Takemoto, et al., Collection of Lectures and Theses presented at 1991 Spring Meeting of Japan Electronic Information Communication Society, vol. 3, pp. 331. As shown in FIG. 8, reference numeral 1 designates a trunk interface; 2, a facsimile signal detector; 3, a sending processor; 4, a DCME interface; 5, a facsimile modem; 6, a receiving processor; and 7, an operations check processor. With this system, if it is inserted between DCME and the trunk circuit and is used in combination with DCME, it is possible to realize high-efficiency transmission of speech/facsimile signals.
In operation, the trunk interface 1 can accommodate trunk lines of digital primary groups of at most nine systems. An input signal of each trunk channel is output to the facsimile signal detector 2 and the sending processor 3 via the trunk interface 1. The facsimile signal detector 2 discriminates whether or not the input signal of each channel is a facsimile signal and outputs the result of discrimination to the sending processor 3. By the result of discrimination, the sending processor 3 outputs the input signal of the channel, which signal has been judged as a speech signal, to the DCME interface 4 as it is, and outputs the input signal of the channel, which signal has been judged as a facsimile signal, to the facsimile modem 5. The facsimile modem 5 demodulates an input voiceband data signal of each channel and outputs the demodulated signal to the sending processor 3. The sending processor generates facsimile signal allocation data and multiplexes the demodulated facsimile signal and the facsimile signal allocation data and outputs it to the FAX route of the DCME interface 4. The DCME interface 4 includes interfaces of digital primary group of nine systems as the speech route and of one system as the FAX route and inputs and outputs speech/facsimile signal from and to the DCME.
At the receiver's system, the signal input via the DCME interface 4 is input to the receiving processor 6 where the demodulated facsimile signal input from the FAX route is output to the facsimile modem 5 based on the facsimile signal allocation data. The facsimile modem 5 modulates the demodulated facsimile signal and outputs the resulting signal to the receiving processor 6. The receiving processor 6 outputs facsimile signals, which are input from the facsimile modem 5, to the trunk interface 1 for the channels occupied by facsimile transmission and outputs signals, which are input from the speech route of the DCME interface 4, to the same trunk interface 1 for the other channels. Then the trunk interface 1 outputs speech/facsimile signals to the trunk circuit.
The operations check processor 7 sets up a facsimile channel for the FAX route in terms of 64000 b/s and sends the set-up data to DCME. The DCME multiplexes the signals, which are input from the speech route, after the high-efficiency speech encoding by the DSI process and the DPCH method and sends the resulting signals and sends the resulting signals, and meanwhile, it continues allocating the individual transmission lines for the facsimile channel of the FAX route. Since the base band data transmission rate of the facsimile signals is 9600 b/s at maximum, which is less than a half, as compared with 32000 b/s or 24000 b/s of the speech signal transmission rate encoded by the ADPCM method, high-efficiency transmission of speech/facsimile signals can be achieved as the effect of DSI is free from being deteriorated by the facsimile signals.
In recent years, 8000 to 16000 b/s speech encoding methods which encode speech signals at lower bit rate than the ADPCM method, have been put to practical use. By combining such low bit rate speech encoding method, DSI technology and facsimile signal base band sending technology, it is possible to realize a circuit multiple transmission system which can transmit speech/facsimile signals at a much higher efficiency.
Now assume that the circuit multiple transmission system equipped with a high-efficiency speech encoder for encoding voice at 9600 b/s accommodates an n-channel trunk circuit. Since the number of speech channels of the transmission circuit can be reduced to about a half the number of accommodated trunk channels by the effect of DSI, it is possible to send voice over the transmission circuit of (n/2).times.9600 b/s. Since the base band data transmission rate of facsimile signals is usually 9600 b/s if the terminal is a GIII facsimile, it is possible to send the facsimile signals to n/2 channels over the transmission circuit of (n/2).times.9600 b/s. However, when sending facsimile signals, it is required to continue allocating the sending side channels during facsimile transmission, unlike speech transmission, from start to end; therefore, assuming that all trunk channels are occupied by facsimile transmission, the transmission circuit requires n.times.9600 b/s. Therefore, with the circuit multiple transmission system equipped with such low bit rate speech encoder, it is difficult to improve the transmission efficiency of voice by DSI. In other words, the transmission circuit capacity is such that it is possible to accommodate telephone line input signals more than those in the case where all input signals are only speech signals.
Another type conventional transmission system is known which is equipped with a high-efficiency speech encoder for encoding voice at two or more bit rates, depending on the load of transmission circuit, so that speech signals can be encoded and sent at a low bit rate if the load is heavy and at a high speed if the load is light. This type system is exemplified by the DCME by ADPCM-DSI method. If facsimile signals, like speech signals, could be sent at 4800 b/s in the case of heavy load of transmission circuit, the foregoing problems would have been overcome. However, since the base band data transmission rate of facsimile signals is decided between the sender's and receiver's facsimile systems during the facsimile communication procedure, it was impossible to transmit facsimile signals at a controlled rate.